A wide variety of medical conditions and disorders have been successfully treated using implantable stimulators. Such implantable stimulators include, but are not limited to, implantable cochlear stimulators, spinal cord stimulators, deep brain stimulators, and microstimulators.
To illustrate, the sense of hearing in human beings involves the use of hair cells in the cochlea that convert or transduce acoustic signals into auditory nerve impulses. Hearing loss, which may be due to many different causes, is generally of two types: conductive and sensorineural. Conductive hearing loss occurs when the normal mechanical pathways for sound to reach the hair cells in the cochlea are impeded. These sound pathways may be impeded, for example, by damage to the auditory ossicles. Conductive hearing loss may often be helped by the use of conventional hearing aids that amplify sound so that acoustic signals reach the cochlea and the hair cells. Some types of conductive hearing loss may also be treated by surgical procedures.
Sensorineural hearing loss, on the other hand, is due to the absence or the destruction of the hair cells in the cochlea which are needed to transduce acoustic signals into auditory nerve impulses. Thus, people who suffer from sensorineural hearing loss are unable to derive any benefit from conventional hearing aid systems.
To overcome sensorineural hearing loss, numerous cochlear implant systems—or cochlear prosthesis—have been developed. Cochlear implant systems seek to bypass the hair cells in the cochlea by presenting electrical stimulation directly to the auditory nerve fibers. Direct stimulation of the auditory nerve fibers leads to the perception of sound in the brain and at least partial restoration of hearing function. To facilitate direct stimulation of the auditory nerve fibers, an array of electrodes may be implanted in the cochlea. A sound processor processes and translates an incoming sound into electrical stimulation pulses applied by these electrodes which directly stimulate the auditory nerve.
Electrical stimulation generated and applied via an implantable stimulator is often implemented in a “monopolar” configuration, in which a relatively remote ground electrode provides the return path for the current delivered by an active stimulating electrode. However, monopolar stimulation produces relatively broad spatial regions of excitation. Depending on the overall stimulation level, such broad excitation patterns can lead to a deterioration in stimulator performance.
Hence, some implantable stimulators are configured to focus or narrow the excitation fields resulting from electrical stimulation by applying compensating current via additional electrodes. Additionally or alternatively, the excitation fields may be narrowed by moving the location of the ground electrode closer to the stimulating electrode. However, these approaches used a “fixed” amount of excitation narrowing across all stimulation levels, and therefore do not provide the optimum solution in terms of stimulator performance.